Understanding the different physiological responses of Malus species under salt stress in the seedling stages will be useful in breeding salt-tolerant dwarfing apple rootstocks. Seedlings of Malus Zumi (Mats.) Rehd. (M. zumi), Malus sieversii (Led.) Roem. (M. sieversii), and Malus baccata (L.) Borkh. (M. baccata) were treated with solution of 0, 0.20%, 0.40%, and 0.60% salinity. Physiological parameters of their leaves and roots were measured at 0 d, 4 d, 8 d and 12 d after salinity treatments. Superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), malondialdehyde (MDA), solution protein (SP), and proline (PRO) initially increased and then decreased. The activities and contents of these parameters were higher in the 0.40% and 0.60% NaCl treatments than in the 0.20% treatment and in the 0% control. M. zumi was the most resistant to salt stress, showing the lowest content of MDA in the leaves and roots, which increased slightly under salt stress. M. baccata had the highest increase in both the content and proportion of MDA. High enzyme activity was shown to play an important role in the salt resistance of M. zumi. Moreover, it can be speculated that there are other substances that also play a major role. We found that osmotic regulation played a key role in response to salt stress for M. baccata even though it was sensitive to salt stress. For M. sieversii, both the osmotic regulation and enzymatic antioxidants were observed to play a major role in mitigating salt stress.

We used 256 apple germplasm resources for a thorough examination of the genetic diversity associated with 26 phenotypic traits (i.e., genetic diversity analysis, cluster analysis, correlation analysis, principal component analysis, and membership function). The average coefficient of variation for 12 morphological traits was 66.39% (21.10–201.5%). The coefficient of variation was highest and lowest for the fruit arris and the width of the eye basin, respectively. Additionally, the diversity index ranged from 0.54 to 1.33. Moreover, the coefficient of variation for 14 numerical traits varied from 5.37% to 50%. The titratable acid content had the highest coefficient of variation, with a diversity index ranging from 2.01 to 2.08 (average of 2.045). A cluster analysis categorized 256 germplasms into four groups, among which Group I included germplasms with large fruits and the best comprehensive performance. Of the top 10 principal components revealed by the principal component analysis, principal component 1 was mainly related to fruit size and flavor. The top 10 germplasms were selected on the basis of comprehensive scores using the membership function method. Furthermore, a stepwise regression analysis identified 15 key traits for identifying apple germplasms, including the vegetative growth day, fruit weight, and the firmness of the fruit without skin. These results can serve as the foundation for future analyses of the phenotypic diversity of apple germplasms, while also providing a theoretical basis for screening, characterizing, and further improving excellent apple germplasms.

Apples are a major horticultural crop worldwide. Grafting techniques are widely utilized in apple production to keep the varieties pure. Interstocks are frequently used in Northern China to achieve intensive apple dwarfing cultivation. High-throughput sequencing was used to investigate differentially expressed genes in the phloem tissues of two different xenograft systems, M (‘Gala’/‘Mac 9’/Malus baccata (L.) Borkh.) and B (‘Gala’/Malus baccata (L.) Borkh.). The results showed that dwarfing interstocks could significantly reduce the height and diameters of apple trees while have few effects on the growth of annual branches. The interstocks were found to regulate the expression of genes related to hormone metabolism and tree body control (GH3.9, PIN1, CKI1, ARP1, GA2ox1 and GA20ox1), these effects may attribute the dwarf characters for apple trees with interstocks. Besides, the interstocks reduce photosynthesis-related genes (MADH-ME4 and GAPC), promote carbon (C) metabolism gene expression (AATP1, GDH and PFK3), promote the expression of nitrogen (N)-metabolism-related genes (NRT2.7, NADH and GDH) in rootstocks, and improve the expression of genes related to secondary metabolism in scions (DX5, FPS1, TPS21 and SRG1). We also concluded that the interstocks acquired early blooming traits due to promotion of the expression of flowering genes in the scion (MOF1, FTIP7, AGL12 and AGL24). This study is a valuable resource regarding the molecular mechanisms of dwarf interstocks’ influence on various biological processes and transplantation systems in both scions and rootstocks.

The breeding of salt-tolerant rootstock relies heavily on the availability of salt-tolerant Malus germplasm resources. The first step in developing salt-tolerant resources is to learn their molecular and metabolic underpinnings. Hydroponic seedlings of both ZM-4 (salt-tolerant resource) and M9T337 (salt-sensitive rootstock) were treated with a solution of 75 mM salinity. ZM-4’s fresh weight increased, then decreased, and then increased again after being treated with NaCl, whereas M9T337′s fresh weight continued to decrease. The results of transcriptome and metabolome after 0 h (CK) and 24 h of NaCl treatment showed that the leaves of ZM-4 had a higher content of flavonoids (phloretinm, naringenin-7-O-glucoside, kaempferol-3-O-galactoside, epiafzelechin, etc.) and the genes (CHI, CYP, FLS, LAR, and ANR) related to the flavonoid synthesis pathway showed up-regulation, suggesting a high antioxidant capacity. In addition to the high polyphenol content (L-phenylalanine, 5-O-p-coumaroyl quinic acid) and the high related gene expression (4CLL9 and SAT), the roots of ZM-4 exhibited a high osmotic adjustment ability. Under normal growing conditions, the roots of ZM-4 contained a higher content of some amino acids (L-proline, tran-4-hydroxy-L-prolin, L-glutamine, etc.) and sugars (D−fructose 6−phosphate, D−glucose 6−phosphate, etc.), and the genes (GLT1, BAM7, INV1, etc.) related to these two pathways were highly expressed. Furthermore, some amino acids (S-(methyl) glutathione, N-methyl-trans-4-hydroxy-L-proline, etc.) and sugars (D-sucrose, maltotriose, etc.) increased and genes (ALD1, BCAT1, AMY1.1, etc.) related to the pathways showed up-regulation under salt stress. This research provided theoretical support for the application of breeding salt-tolerant rootstocks by elucidating the molecular and metabolic mechanisms of salt tolerance during the early stages of salt treatment for ZM-4.

Apple (Malus Mill.) is one of the most important fruits in China, and it boasts the world’s largest cultivation area and yield. It needs to be grafted onto rootstocks to maintain a variety of characteristics. China has many apple rootstock resources that exhibit high resistance and strong adaptability; for these reasons, they are highly suited to China’s complex and diverse natural environment. In China, apple rootstock breeding began in the 1970s, and now, several rootstocks, such as the ‘GM256’ and ‘SH’ series, are widely used. However, domestic rootstock resources and varieties are difficult to root. This affects the selection, utilization, and promotion of apple rootstocks. Cutting is an important method of rooting for apple rootstocks. This study discusses the main factors that affect rooting in rootstock cutting propagation; it also summarizes the rooting ability of different apple rootstocks and presents analyses of the demand for rootstocks in the major areas of apple production in China. We present the apple rootstock resources that are suitable for the soil and climate conditions of this production. We also call for research on the cutting roots of these specific apple rootstock resources to be expanded and strengthened. It is hoped that cutting rootstocks suitable for major areas of apple production may be more easily screened and bred successfully in China in the near future.

‘Huahong’ is a popular apple cultivar because of its anti-browning properties and appealing aroma and flavor. It is mainly planted by grafting on dwarf interstocks in Northeast China. We investigated the different aroma profiles of apple fruits grown from six dwarf interstocks (‘CG24’, ‘SH38’, ‘SH3’, ‘MD001’, ‘Mac9’, and ‘CX5’) and from no interstocks (CK). A total of 55 VOCs were detected, including esters (25), aldehydes (14), alcohols (8), ketones (3), alkane hydroxyls (3), and acids (2). Among the VOCs, 48 were detected in the skin and 21 in the pulp. The skin of ‘Huahong’ apples had a strong sweet aroma, and the pulp was green with a subtle aroma. The dominant compounds (>5% of total content) in the skin were 2-methyl butyl acetate, hexyl 2-methyl butyrate, caproic acid butyl ester, hexanal, (Z)-2-heptene aldehyde, and 6-methyl-5-heptene-2-ketone, while in the pulp, they were 2-methyl butyl acetate, methanol, 2-methyl-1-butanol, hexanol, and hexane. Compared with CK, ‘SH38’, ‘MD001’, and ‘SH3’ interstocks had increased total aroma content, and ‘CX5’ and ‘CG24’ had suppressed aroma. The effects of interstocks on aroma were mainly reflected in skin. The VOC content ranged from 3297.52 to 9895.75 µg·kg⁻¹ in skin, and from 748.62 to 1369.21 µg·kg⁻¹ in pulp. PCA revealed that use of interstock ‘SH38’ mainly affected esters. ‘MD001’ affected hexane and 4-pentene-1-acetate; ‘Mac9’ and ‘SH3’ affected octanoic acid-2-methyl butyl ester, hexyl butyrate, and 2-methyl-1-butanol; and ‘CX5’ and ‘CG24’ had a greater impact on isoamyl propionate and 1-pentene-3-ol. Finally, ‘SH38’ had the highest principal comprehensive score. ‘SH38’ and ‘SH3’ interstocks resulted in significantly increased apple VOC content.